(+)-7′,7′-Dimethyl-5-hydroxy-2R,3S-trans-pubeschin from Entandrophragma cylindricum

0031-9422@4)E0247-P

Perjpmon

Phyewhtmutry. Vol. 37. No. 2. pp. 529-531. 1994 copyright 0 I994 Ekner sama Lid Rnted m CRPI Britain. All rights mened MII- 9422/w 17.00+000

(+)-7’,7’-DIMETHYL-5-HYDROXY-2R,3S-TRANS-PUBESCHIN

FROM

ENTANDROPHRAGMACYLINDRICUM DAVID NGNOKAM, GEORGES MAWOT,

JEAN-MARC NUZILLARD and EnENNE TSAMO+

Laboratoire de Pharmacognosie U.R.A. au C.N.R.S. 492. Facultt de Pharmacie. Universk

de Reims. 51. Rue Cognac+Jay 51096

Reims. France; *Department of Organic Chemistry. University of Yaounde 1. Box 812 Republic of Cameroon (Recenvd 7 Fehruury Key

Word

Index-Entandrophrogma

hydroxy-2R,3S-rrans-pubeschin;

Abstract--(

heartwood partial

cylindricum;

sapele; catechin;

1994)

Meliaceae; peltogynoids.

heartwood;

(+)-7’,7’-dimethyl-5-

+ )-7’,7’-Dimethyl-S-hydroxy-2R,3S-[runs-pubeschin was isolated from the methanol extract of the of Entandrophragma cylindricum. Its structure was determined on the basis of spectroscopic data and a

synthesis from

( + )-catechin.

INTRODUCTION

RESULTS AND DISCUSSIONS

The genus Entandrophragma is widespread on the African continent, south of the Sahara, and occupies a prominent position in traditional medicine. Five species are known in Cameroon, and among these, Entandrophragmu cylindricum Sprague (locally known as sapele) has been the object of limited investigations. Our previous contribution reported the isolation of three new squalene triterpenoids [ 11. In an extension of our studies. the methanol extract of the wood of Enrandrophragma cylindricum was examined in greater detail for minor concentrations of the novel peltogynoid: ( + )-7’,7’-dimethyl-S-hydroxy-2R,3Strans-pubeschin (1).

Compound I. CIBH,806r mp 198. 199’. was a fine powder. Its UV spectrum showed two maxima absorptions at 282 and 219 nm. characteristic of catechins [2]. The addition of boric acid in the presence of sodium acetate induced a bathochromic shift of about 6 nm which confirmed the presence of the orthohydroxyl groups. Its IR spectrum showed no carbonyl absorption. The “C NMR spectrum displayed signals for 18 carbons: two methyls, one methylene, two methines beating oxygen, four sp2 methines and nine quaternary carbons including five sp2 bearing oxygen. three sp2 and one sp’. This was confirmed by the observation in the ‘H NMR

HO

H

OH

OH 1: (+)-7’. 7’dimetbyl-5-hydroxy-2R.

3S-rrunr-pubeschin

3: (+)-2R. 3S-rran.r~pubeschin

530

D. NGNOKAM et a/. Table I. ‘H and ‘“CNMR

spectral data and HMBC correlations of compound I

_, (Hz)

5,

J,,, and J,

2 3

73.5 66.6

4 5 6 7 8 9 10 I’ 6

27.2 157.2 95.2 156.7 95.9 156.7 100.1 124.7 1355

Jd(4.5)5,.,=9.2,5 >_>. =0.7ddd(3.9) J3_z=9.2,J,_ .,,,, =10.4. J,_,,,,,=6.0dd(2.5.3.0) J,., = 15.5. J, ,,,, _, = 10.4. J,,,,., =6.0

5’ 4

112.1 145.0

3’ 2’ 7’

143.9 111.8 75.5 31.4 28.7

Me-7’ MC-~’

d (6.0) J,_, = 2.5 d(6.1) J,.,=2 5

H-4 and H-2’ H-2 and H-4 H-2 H-4 and H-6 H-8 H-6 and H-8 H-6 H-4 and H-8 H-4, H-6 and H-8 H-2 H-2, H-2’ and 7’-Me

.s(6.7) H-5’ and H-2’ H-5’ and H-2 d(7.1) J, _,=0.7 S(l.5) .\(1.6)

of two singlet methyls, one methylene and six methines. The molecular formula C,sH,s06, [Ml’. at m/z 330. involves IO degrees of unsaturation. The presence of 12 sp2 carbon atoms which were assigned to six double bonds, suggests that the four remaining unsaturations made up a tetracyclic compound. Comparison of ‘H and “C data (Table I) and those of 2R,3S-rrons-pubeschin (3) and elaeocyanidin (4) [3,4] allowed the determination of the structure of 1 as 7’,7’-dimethyl-5-hydroxy-2R.3Srrans-pubeschin. This structure agreed with the mass spectrum where intense peaks were observed at m/z 330 312 [M-H,O]. 192 and 139 WI’ t 315 [M-Me], (retro-Diels-Alder fragmentation of the chroman heterocyclic nucleus. The HMBC spectrum (see connectivities in Table I) also agreed with the proposed structure 1. An alternative structure in which the hydroxyl groups in the ring A are placed at C-6 and C-8. instead of C-5 and C-7 was rejected on the basis of biogenetic considerations [5] and partial synthesis of 1 from catechin (2). The location of the hydroxyls at positions C-3’ and C-4’ was based on a COSY experiment which showed no correlation between H-5’ and H-2’. consequently placed in the paru position. Since the observed J, J was 9.2 Hz. it can, therefore. be concluded that H-2 and H-3 are truns diaxial. In order to determine the absolute configuration of C-2 and C-3, 1 was prepared in a single step by the condensation of( + )-catcchin (2) with 2,2-dimethoxypropane in the presence of p-toluene sulphonic acid as a catalyst. Synthetic material is in all respects identical with the natural product. including specific rotation. Compound I is a dimethylated homologue of the peltogynoids, a series of compounds actively investigated during the mid-1970s [6, 73. The origin of the additional carbon atom in the peltogynoids is still an object of qectrum

HMBC correlations C correlates with

C

H-5’ and 7’-Me 7’-Me 7’-Me

debate and it has long been thought that it derives from a methoxyl through an oxidative pathway [8-l 11. The isolation of the title compound shows that the homologation of catechin is not limited to one carbon atom. The synthesis of 1 from 2 and an acetone equivalent demonstrates that the hypothesis of formation by condensation of a catechol with a carbonyl group is not contradicted by the chemical reactivity of the partners.

EXPERIMENTAL

General. Mp: uncorr. IR: KBr. NMR spectra were recorded at 75.4 MHz for “C and 300.1 MHz for ‘H. Chemical shifts are given in 6 values (ppm) with TMS as int. standard. The 13C chemical shifts are summarized in Table I. El-MS: 70 eV. TLC was carried out on silica gel. Compounds were detected by spraying with 50% solution of H,SO, in Hz0 followed by heating. UV spectra were determined as methanol solutions. Plant material. The heartwood of Entandrophragma cylindricum was collected from Ebolowa (Cameroon). A voucher specimen is deposited at the National Herbarium, Yaounde. Extracrion and isolation. The air-dried finely powdered wood of Enrandrophragma cylindricum ( 5 kg) was extracted with MeOH (10 I) at room temp. The MeOH extract (185 g) was stirred with CH,CI, at room temp. to give a solid and a liquid. The liquid phase was filtered in vucuo over a scintered glass filter and the solid (100 g) was recovered. Part of this solid (I5 g) was chromatographed on silica gel column using a CH,CI,-MeOH gradient and collecting 100 ml frs. Frs 52-57 (100 mg) were collected and chromatographed repeatedly on silica gel using the same solvent system to yield 1 (7 mg).

Peltogynoid from Enrandrophraytnacylmdricum

(+)-7’,7’-Dimethyl-S-hydroxy-2R,3S-trans-pubeschin requires: C: 65.5, H: 5.5.0: 29.0). [a]u + 149 (MezCO; c 0.10). UV: A”,:‘” nm: 282, 219; MeOH + B(OH), + CH,CO,Na: 282,213. ‘H and r3CNMR (Me&O-d,) see Table 1. EIMS m/z (rel. int.): 139 (50). 174 (60), 177 (51). 192 (5) 203 (40). 315 (100). 330 (38). Preparation of compound 1. Compound 1 was prepared by dissolving (+ )-ZR,)S-catechin (2) (1 g) in Me,CO (10 ml), and adding 2,2_dimethoxypropane (10 ml) and a catalytic amount of p-toluene sulphonic acid. The reaction was complete in 6 hr with no trace of the starting catechin. Et,0 was added and the organic phase washed with NaHCO, and dried over NazSO,. Solvent removal gave a mixture which was chromatographed on a silica gel column using CH,CI,-MeOH gradient and collecting 5 ml frs. Polar frs 6-7 were collected (45 mg) and repeatedly chromatographed on silica gel using the same solvent system to yield a compound (15 mg), identical with the natural product (CCM, ‘H NMR, and [a]n). (1). (Found, C: 65.4, H: 5.5, 0: 29.1, C,aH,,O,

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